Move camera to existing camera (with keyframes)
You can use Constraints for this.
Consider you have three 'Target' cameras : Target1, Target2, Target3 - and one 'MoveableCamera'. On the MoveableCamera, add a Copy Location constraint for each of the targets (1,2,3) and also a Copy Rotation constraint for each of the targets. In each case set the 'Target' object to the respective target camera.
All that's then required is to adjust the Influence of each constraint to move the MoveableCamera towards each of the locations and rotations that you desire. The Influence can be keyframed as for other properties.
For example, to move the camera to Target1, simply set the Target1 influence to 1.0 and the others to 0.0. To move between two targets simply set the first (in the constraint list) to 1.0 and then vary the second towards 1.0, depending on how close to that target you want.
For example, to move the MoveableCamera to 1/3rd of the way between Target2 and Target3, set Target2's Influence to 1.0 and Target3's Influence 0.333.